What's Causing School-/Mass Shootings?

Breaking down the plausible deniability puzzle of mass shootings. I excluded the Muslim ones* all-together, as well as FBI/CIA/Deep state guided or allowed ones, and the gang related ones mostly.

*You know - Muslim comes to West, engages in some "sinful" behavior, gets a bit more religious, finds out martyrdom is a sure way to heaven even after the sinful life - Terror attack.